In recent years, following high integration of semiconductor products as seen in semiconductor memories, VLSIs (very large scale integrated circuits), and so forth, there have been required fine patterns that exceed a transfer limit of the photolithography process. In view of this, in order to enable transfer of the fine pattern, there has been proposed an extreme ultraviolet lithography process (EUV lithography process) using an extreme ultraviolet light with a shorter wavelength. Herein, the EUV light represents a light in a wavelength band of a soft X-ray region or a vacuum ultraviolet region, specifically, a light with a wavelength of about 0.2 to 100 nm.
As described in, for example, Japanese Patent Application Publication (JP-A) No. H8-213303, an EUV reflective mask for use in the EUV lithography has, on a substrate of silicon, quartz, or the like, an EUV (extreme ultraviolet light in the soft X-ray region having, for example, a wavelength of about 13.4 nm) multilayer reflective layer, a buffer layer thereon, and further thereon, an EUV absorber layer formed in a pattern. The buffer layer is provided between the EUV multilayer reflective layer and the EUV absorber layer for the purpose of protecting the EUV multilayer reflective layer during a pattern forming process of the absorber layer and a repairing process thereof.
Light incident on the reflective mask in an exposure apparatus is absorbed at a portion where the absorber pattern exists, while an image reflected by the multilayer reflective film where no absorber pattern exists is transferred onto a wafer through a reflective optical system.
Referring to FIG. 4, description will be given about a manufacturing method of a reflective mask for use in the EUV lithography. FIG. 4 is a schematic sectional view showing a conventional reflective mask manufacturing process.
(1) On a SiO2 substrate 11 are formed, in order, a layered film 12 being an EUV multilayer reflective film, a buffer layer 13 thereon for the purpose of protecting the EUV layer in an absorber pattern forming process, and an absorber layer 14 being an EUV absorber thereon. Thereafter, an organic resist film 16 is applied to the surface of the absorber layer (FIG. 4(1)).
(2) The organic resist film 16 is formed with a resist pattern 16a (FIG. 4 (2)).
(3) The absorber layer 14 is etched by the use of the resist pattern 16a to thereby form an absorber pattern 14a having a predetermined pattern. Here, the formed absorber pattern 14a is subjected to inspection (FIG. 4(3)). Specifically, detection is made of a pinhole defect 21 (also called a white defect) formed by removal of the absorber layer at a portion that should not be removed by etching and a portion 22 (also called a black defect) where the absorber layer is not sufficiently removed due to insufficient etching.
(4) The defects of the absorber pattern 14a are corrected to thereby form a corrected absorber pattern 14b (FIG. 4 (4)).
(5) Finally, a pattern 13a of the buffer layer 13 is formed to thereby obtain an EUV reflective mask. When an EUV light 31 is irradiated thereto, the EUV light 31 is reflected only at a portion where the reflective layered film 12 is exposed by removing the absorber layer 14 and the buffer layer 13, and can be used for the lithography (FIG. 4 (5)).
In the foregoing EUV reflective mask producing process, use is made of a reflective inspection device using a far ultraviolet light having a wavelength of 193 nm to 257 nm in the inspection of the absorber pattern at (3).
The inspection is performed by applying this inspection light to the surface of the reflective mask formed with the pattern and by observing a contrast of reflection of the inspection light on the surface of the mask.
Further, after the removal of the buffer layer at (5), an inspection is carried out to finally confirm whether or not the absorber pattern is formed according to a specification. This final inspection of the pattern is also conducted by, like the foregoing inspection, using the far ultraviolet light as the inspection light and observing a contrast of reflected light of the inspection light on the surface of the mask.
In the foregoing formation of the transfer pattern in the absorber layer, use is made of the resist made of an organic substance. However, such a resist generally has a low resistance against dry etching so that the resist film is damaged during the formation of the pattern of the absorber layer. Therefore, the resist layer is required to have a certain or greater thickness (normally about 500 nm to 800 nm).
On the other hand, as the line width of a pattern required for a mask is reduced, it becomes difficult to form a fine pattern in such a thick resist in view of the following points.
Specifically, first, it is difficult to ensure the accuracy in shape in a vertical direction of the resist so that the shape accuracy of an absorber pattern is degraded. Secondly, an etching gas is reluctant to advance into narrow paths of the resist pattern and a gas generated by etching is also liable to stay therein so that the etching reaction is difficult to proceed at portions where the line width is narrow. Consequently, a difference is caused between etching speeds at a portion where the line width is broad and at a portion where the line width is narrow so that uniform etching cannot be achieved in the plane of the mask.
From this point of view, there has been a problem that when the conventional thick resist is used, it is difficult to form a pattern with a fine line width, for example, a resolution of 0.1 μm or less.
On the other hand, in the foregoing inspection of the pattern of the absorber layer, the inspection using the foregoing inspection light is performed between the surface of the buffer layer exposed at a portion where the absorber layer is removed or the surface of the multilayer reflective film exposed by the removal of the buffer layer, and the surface of the absorber layer at a portion where the absorber layer remains.
Therefore, there has been a problem that if a difference in reflectance with respect to the wavelength of the inspection light is small between the surface of the buffer layer or the surface of the multilayer reflective film and the surface of the absorber layer, the contrast in the inspection becomes poor so that the accurate inspection cannot be achieved.
In view of this, the present invention has been made for solving the foregoing problems and has an object to obtain a reflective mask and a reflective mask blank wherein a fine pattern can be formed in an absorber layer with high accuracy in shape, a sufficient contrast can be achieved in a pattern inspection, and a pattern transfer with high accuracy is enabled.